The present invention relates to an offshore tanker loading system. Prior to the present invention, single point mooring (SPM) terminals have been used. These terminals were normally in relatively unsheltered waters and allowed the tanker to be aligned with the main weather direction much like a weather vane. The mooring hawser plays a key part in the above-described situation, as this element effectively ties the tanker to the SPM terminal while carrying all the environmental loads and allowing for all wave induced motions of the vessel. Further, for all intents and purposes, the tanker has, while it remains moored, become a "dead" ship
Modern technology has made it possible for many ships to stay on a specific offshore location by means of their propulsive systems. These propulsive systems have to meet certain requirements, such as output thrust and power should be continuously variable in magnitude and direction. Also, the propulsive system is tied to a data acquisition system which monitors and measures the magnitude and direction of wind, current and waves, as well as the actual position of the ship. The direction and magnitude of the propulsive thrust is adjusted on a continuous basis to counter the continuously varying environmental forces which act on the ship and which would normally urge it to move off its desired location. Ships fitted with such a propulsive system are referred to as Dynamically Positioned (DP) ships.
In the offshore oil industry, such ships are of great importance because they allow a quick turn-around time in loading or unloading their cargo, as no SPM operation needs to be performed. These SPM operations are time consuming and sometimes, due to weather, even impossible. The DP tanker, however, requires a flexible loading system for its cargo and the present invention is directed to such loading system.
An example of such loading system is disclosed in the paper OTC 5747 given at the Offshore Technology Conference in Houston, Texas, May 2-5, 1988 by K. Mork, Ugland Engineering A/S and entitled "Stratfjord `A` Offshore Loading System (UKOLS) " This system has been used but has a number of disadvantages. The main disadvantage is that, once the tanker loading operation is completed, the hose has to be laid back onto the sea floor. If the hose termination normally attached to the tanker were left afloat, the overall geometry of the system could lead to entanglement of the various parts Subsea swivels could alleviate some of this risk. Another disadvantage is the fact that the "operating area" of the tanker, when it is connected to the UKOLS, is described by a ring-shaped area. The center area of the ring is a "no-go" area for the tanker, as it would mean that the portion of the hose connected to the tanker would be in continuous contact with the subsea buoy. This contact results in chafing of the hose portion contacting the buoy. One other disadvantage to the ring-shaped operating area, when compared with a full-circle operating area, is that when wind or current change direction, more energy is required to keep the ship within the operating area. This is due to the fact that not only a change of the ships heading has to be achieved, but also a lateral displacement. If the operating area is a full circle, one could position the tanker in the center and merely change the heading when current and wind dictate such change.
Another mooring system is described in the patent application in Great Britain GB 2.239.441A. This system requires the use of a powered turntable on the tanker in order to overcome internal friction loads in the fluid swivel arrangement. Also, the complexity of the hook-up operation, which requires the load to be transferred from an initial pull-in line to a final hook-up line, is considered to be a difficult and hazardous operation to personnel.